In the art of separating mixtures of organic compounds, it is relatively easy to separate such mixtures into fractions having specified boiling ranges by fractional distillation and the like. However, it is substantially more difficult to separate such mixtures into individual chemical structural types of compounds of fractions enriched in particular structural compounds. This, of course, is due to the fact that many structurally different compounds have the same or nearly the same boiling points. Consequently, it has become common practice to separate structurally different compounds of a mixture by employloying solvent extraction, utilizing solvents which are selective for compounds having a particular type or types of chemical structure from other compounds having different chemical structures. One such group of solvents is organic sulfoxides. While organic sulfoxides have heretofore been suggested as a general solvent for separations of hydrocarbons, oxyorganic mixtures and the like, the commercial use of such solvents has been quite limited. This limited use of organic sulfoxides, as solvents, is due to the substantial expense of utilizing such solvents, primarily because large volumes of solvent are required and effective means for recovering the solvent for reuse have not been particularly effective or are highly energy intensive. This is particularly true where both the extract fraction and the raffinate fraction are normally liquid materials.
The above-mentioned problems are particularly prevalent in the petroleum refining art. Crude oils can be grossly separated into fractions having predetermined boiling ranges, for example, normally gaseous hydrocarbons, crude naptha, heavy naptha, middle distillates, gas oil fractions and tar fractions. However, in most cases, these various fractions must be subjected to further thermal and catalytic conversion processes in order to produce useful products. For example, the napthas are treated and/or converted to gasoline and solvent products; the middle distillates to kerosene and furnace oils and the gas oil and tar to lighter products which, in turn, must be further treated or converted to end products. Most such conversion processes, in turn, create their own problems of separating the materials into useful end products, such as gasolines, furnace oils, diesel oils and chemical process feedstocks. For example, gas oil fractions of crude oil are generally subjected to thermal or catalytic cracking in the presence of an acidic catalyst such as silica-alumina. Catalytic cracking produces a gas fraction, a naptha fraction or fractions, a light fuel oil, commonly referred to as light cycle oil, and heavy gas oils, commonly referred to as heavy cycle oil. While fractional distillation can be utilized to separate catalytic cracking products, in accordance with the boiling point range, in order to produce the most useful and valuable products, it is necessary to further separate some of these materials into fractions enriched in hydrocarbons having particular chemical structures. The present invention is particularly related to the separation of light cycle oil into more useful and valuable products.
Organic sulfoxides, particularly dimethylsulfoxide, have heretofore been utilized for separation of light cycle oils into more valuable component fractions. However, such extraction processes are fraught with the same problems, previously mentioned, with respect to the separation of normally liquid organic compounds with organic sulfoxides, namely, the requirement of substantial volumes of organic sulfoxide and inefficient or highly energy intensive techniques for recovering the solvent. In such processes, it is conventional practice to utilize vacuum distillation, azeotropic distillation and the like to recover the organic sulfoxide from the dissolved extract fraction. However, such distillation techniques are highly energy intensive and significant amounts of solvent are carried over with the extract fraction. It would, therefore, be highly desirable to provide an improved technique for separating the organic sulfoxide from the extract fraction and, particularly, to provide such a process which reduces the energy requirements of separation.